90 F. M. BALFOUR. 



ventral thickening of the embryo now occupies rather more 

 than half the circumference of the ovum. 



Transverse sections show that considerable changes have 

 been effected in the constitution of the blastoderm. In the 

 previous stage, the ventral plate was formed of an uniform 

 external layer of epiblast, and a continuous internal layer 

 of mesoblast. The mesoblast has now become divided along 

 the whole length of the embryo, except, perhaps, the proce- 

 phalic lobes, into two lateral bands which are not continuous 

 across the middle line (PL IX, fig. 13 me). It has, more- 

 over, become a much more definite layer, closely attached 

 to the epiblast. Between each mesoblastic band and the 

 adjoining yolk there are placed a few scattered cells, which 

 in a somewhat later stage become the splanchnic mesoblast. 

 These cells are derived from the yolk-cells ; and almost 

 every section contains examples of such cells in the act of 

 joining the mesoblast.. 



The epiblast of the ventral plate has not, to any great 

 extent, altered in constitution. It is, perhaps, a shade 

 thinner in the median line than it is laterally. The division 

 of the mesoblast plate into two bands, together, perhaps, 

 with the slight reduction of the epiblast in the median 

 ventral line, gives rise at this stage to an imperfectly 

 marked median groove. 



The dorsal epiblast is still formed of a single layer of flat 

 cells. In the neighbourhood of this layer the yolk nuclei 

 are especially concentrated. The yolk itself remains as 

 before. 



The segments continue to increase regularly, each fresh 

 segment being added in the usual way between the last 

 formed segment and the unsegmented caudal lobe. At the 

 stage when about nine or ten segments have become estab- 

 lished, the first rudiments of appendages become visible. 

 At this period (PI. VIII, fig. 4) there is a distinct median 

 ventral groove, extending through the whole length of the 

 embryo, which becomes, however, considerably shallower 

 behind. The procephalic region is distinctly bilobed. The 

 first segment (that of the chelicerse) is better marked off 

 from it than in the previous stage, but is without a trace 

 of an appendage, and exhibits therefore, in respect to the 

 development of its appendages, the same retardation that 

 characterised its first appearance. The next five segments, 

 viz. those of the pedipalpi and four ambulatory appendages, 

 present a very well-marked swelling at each extremity. 

 These swellings are the earliest traces of the appendages. 

 Of the three succeeding segments, only the first is well 



